CALIFORNIA – In California’s Central Valley, a group of fruit growers picks their way to a particular plum tree. A field hand uses a pole to divest the tree of a piece of fruit and hands it to his boss, Leith Zaiger Gardner. She slices it and presses a juicy chunk against the screen of what looks like a high-tech ear thermometer. Fruit juice drips from the screen, and the device—a refractometer, a tool that uses light to measure sugar levels—registers 19 brix.

“Nineteen brix! Tasty!” shouts Gardner, general manager of Zaiger Genetics, a family-run business located just outside of Modesto, California, and the foremost global developer of plums, nectarines, peaches, cherries and just about every combination of those fruits that you can imagine.

Gardner’s father, Floyd Zaiger, the head of Zaiger Genetics, approves.

“I don’t get tired of plums as long as they’re sweet,” he says. “But it might take 300 to 400 tries to get a good one. It’s like a puzzle. You can’t put it down until you’re finished.”

Zaiger started playing with these puzzles nearly 60 years ago, when he began genetically hand-crossing stone fruits at his commercial nursery as a hobby. Gradually, he made a series of innovations: tracking each fruit’s lineage with a new and improved system, starting seedlings in movable containers and greenhouses, lengthening the time he had each season to make crosses and increasing the number of crosses that he could make.

In short order, Zaiger’s weekend pastime grew into a world-famous (in fruit circles) concern that sees growers and nursery managers from around the world making pilgrimages to the Modesto orchard. Today, two of California’s largest plum growers have stopped by, as well as a grower from Australia. If you’ve eaten a Pluot (a plum-apricot with more plum than apricot characteristics) or Aprium (the same but with more apricot), you’ve experienced firsthand the results of Zaiger’s obsession with stone fruits, which persists to this day. Though he’s nearing 90, Zaiger still works seven days a week, using a golf cart to inspect his trees.

“My legs don’t keep up the way they used to, but every day something pops up that shouldn’t happen,” he says. “Sometimes it’s good; sometimes it’s not.”

In the works, says Zaiger, is the release of the Peacotum—a peach-apricot-plum said to taste like fruit punch—and the Pluerry, a plum-cherry hybrid.